Answer and Explanation:
The narrator feels that the police are making fun of him because they are talking and laughing among themselves. While the narrator is very disturbed by a random noise that is driving him crazy. The policemen do not notice the noise, but the narrator is so disturbed that he screams, squirms and begs for the noise to stop, which makes the police think his behavior is funny and think it is funny.
The "horror" to which the narrator refers is a high-pitched and deafening noise, which is making him paranoid, but only he hears it, so no one believes him.